1. Field of the Invention
The present invention relates to an improvement of an apparatus for controlling an electric vehicle using an induction motor, and particularly relates to an apparatus for controlling an electric vehicle in constant speed operation.
2. Description of the Prior Art
As to constant speed running of an electric vehicle using an induction motor, Japanese Patent Unexamined Publication No. 48-12418 discloses a control apparatus for an electromobile for running on a road.
That is, the control apparatus is arranged such that a slip frequency is determined by a function generator so that the slip frequency changes linearly in proportion to a deviation of an actual speed of an induction motor from a required speed with positive and negative limitations. Being not different from the general ones, the other control systems are arranged to control the output voltage of an invertor in response to a frequency command applied to the invertor.
Further, Japanese Patent Unexamined Publication No. 54-22607 discloses a control apparatus for a railway electric vehicle in which the output frequency of an invertor is controlled in response to various speed commands which are set correspondingly to respective notch commands, for example, such that 10 km/h, 30 km/h, 60 km/h, and 100 km/h are set respectively correspondingly to the first, the second, the third, and the fourth notches.
The magazine "Science of Electric Vehicle" Vol. 38, No. 2, 1985, pages 15 through 24, particularly page 20, discloses constant speed control which is attained by reducing the slip frequency when the speed of an electric vehicle approaches a limit speed.
In those control methods, however, there are disadvantages that the two former ones are much different in handling of operation of the railway electric vehicle from the conventional one and that the latter one is insufficient in freedom in the constant speed running.